Recently, with high integration and large capacity of a Large Scale Integration (LSI), a circuit dimension required for a semiconductor element becomes increasingly narrow.
Using an original image pattern (that is, a mask or a reticle, hereinafter collectively referred to as a mask) in which the circuit patterns are formed, a reduction projection exposure apparatus called a stepper exposes and transfers the pattern onto a wafer to form a circuit, thereby producing the semiconductor element. A charged particle beam writing apparatus using a charged particle beam, for example, the electron beam writing apparatus using an electron beam is used to produce the masks for transferring the fine circuit patterns onto the wafer. In the electron beam writing apparatus, as one example of a charged particle beam writing apparatus, because the electron used is a “wave” having an extremely short wavelength, resolution proportional to the wavelength of the beam can be enhanced, and the apparatus can be used to produce a highly accurate original pattern.
Japanese Laid-Open Patent Publication No. Hei 09-293670 (1997) discloses a variable shape electron beam writing apparatus used for electron beam lithography technique. Pattern writing data for such an apparatus is prepared by applying processing of the design data (CAD data) of a semiconductor integrated circuit designed by a CAD system, such as correcting the design data and dividing the graphic pattern included in the design data, as some examples of design data processing.
For example, the dividing process divides the graphic pattern into units of the maximum shot size, which is defined by the size of the electron beam. In addition, the apparatus sets the coordinate positions, size, and the radiation time of each divided shot. The pattern writing data is then produced so that shots are shaped accordance with the shape and size of the graphic pattern to be written. The pattern writing data is divided into pieces each corresponding to a strip-shaped frame (or main deflection region), and each frame is divided into pieces each corresponding to a sub-deflection region. That is, the pattern writing data for the entire semiconductor chip has a hierarchical data structure consisting of data of each of a plurality of strip-shaped frames in accordance with the size of the main reflection regions, and data of each of a plurality of units in accordance with the size of the sub-reflection regions (smaller in size than the main deflection regions) in the frame.
In the above-mentioned sub-deflection region, the electron beam is scanned at a higher speed than it is scanned over each main deflection region by the sub-deflector, which is one deflector included in an electron beam writing apparatus. The sub-deflection regions are generally the smallest units for a writing operation. When a writing operation is performed in each sub-deflection region, the shaping deflector forms shots of a size and shape in accordance with graphic patterns to be written. Specifically, in the electron beam writing apparatus, the electron beam emitted from the electron gun is shaped into a rectangular shape by a first aperture and then projected to a second aperture by the shaping deflector, resulting in a change in the shape and size of the beam. The electron beam is then deflected by the sub-deflector and the main deflector, and irradiated onto the mask mounted on the stage which is provided downstream of the electron gun.
It is well known that, when a writing operation is performed using the electron beam writing apparatus, a displacement of a position irradiated with the electron beam is generated, in accordance with the elapsed time and thus degrades the writing pattern, for example. The displacement of the position irradiated with the electron beam is called a beam drift. For example, the following phenomenon can be listed as a cause of the beam drift.
A trace amount of hydrocarbon (CnHm) gas is contained in the electron beam writing apparatus that has been substantially evacuated to allow formation of the electron beam. A component and a resist in the apparatus can be listed as an example of the gas generation source, and it is difficult to completely eliminate the gas from the gas generation source. The irradiating electron beam (or scattered electron) reacts with the gas to form contaminants on a surface of the component such as a deflector in the apparatus. When charges are accumulated on the contaminants, an electric field is generated by a difference in accumulated charge amount, and the irradiating electron beam is deflected by the electric field. As a result, the position irradiated with the electron beam is displaced.
For example, as disclosed in JP 09-259811 A, a method for cleaning the electron beam writing apparatus using ozone (O3) is well known as a method for reducing the contaminants in the apparatus to solve the problem of the displacement of the position irradiated with the electron beam. In the method, ozone gas is introduced into the electron beam writing apparatus, and the ozone is caused to react with the contaminants to change the contaminants to volatile gas, thereby removing the contaminants.
In the method for introducing the ozone to remove the contaminants in the electron beam writing apparatus, while the electron beam writing apparatus is operating, the ozone gas can be injected into the apparatus to remove the contaminants. That is, a collision between the ozone and the electron beam is caused in the apparatus, and the ozone is separated into oxygen (O2) and active oxygen (O*). For example, the separated active oxygen is caused to react with the contaminants adhering to the mask or the surface of each component in the apparatus, and the contaminants are evaporated as carbon monoxide gas (CO), carbon dioxide gas (CO2), and water (H2O), etc. In the conventional contaminant removing method, the electron beam writing apparatus can be kept clean without replacing the component, to which the contaminants adhere due to disassembly of the apparatus. That is, in-situ cleaning of the electron beam writing apparatus can be performed by the method.
However, for the conventional electron beam writing apparatus, the position irradiated with the electron beam is displaced by a slight change in the ozone gas in the apparatus. Therefore, in the conventional method, it is difficult to suppress degradation of the writing pattern to perform a highly accurate writing operation.
For example, for the conventional method in which the ozone gas is introduced into the electron beam writing apparatus, it is found that the position irradiated with the electron beam is displaced depending on a pressure of the gas introduced into the apparatus. Although control of the pressure of the introduced gas is demanded with high accuracy, it is difficult to control the pressure in the conventional control technology using a valve is used. It is also found that the change in irradiated position due to the pressure of the introduced gas depends on a measuring method, specifically based on whether an irradiated target is a calibration substrate or the mask. In this case, the irradiated position during adjustment differs from the irradiated position during the actual writing operation, and the writing operation is barely performed at the desired irradiated position.
The displacement of the position irradiated with the electron beam due to the introduction of the ozone gas is attributed to the fact that the ozone gas introduced into the electron beam writing apparatus or a decomposition product of the ozone gas is ionized by the irradiation with the electron beam and therefore a positive ion is formed by the ionization. That is, the positive ion formed from the ozone gas remains on an optical path of the electron beam to form the electric field around the optical path, thereby exerting a lens effect.
Accordingly, there is a demand for the electron beam writing apparatus and the electron beam writing method, in which a variation of an influence caused by the ozone introduction can be suppressed to a lower level to stably perform the highly accurate writing operation while the in-situ cleaning is performed by the ozone introduction to eliminate the influence of the contaminants. The demand is not limited to only the electron beam writing apparatus and the electron beam writing method, but also a charged particle beam writing apparatus and a charged particle beam writing method in which other charged particle beams such as an ion beam are used. That is, there is the demand for the charged particle beam writing apparatus and the charged particle beam writing method, in which the variation of the influence caused by the ozone introduction can be suppressed to a lower level to stably perform the highly accurate writing operation while the in-situ cleaning is performed by the ozone introduction to eliminate the influence of the contaminants.
An object of the present invention is to provide a charged particle beam writing apparatus that stably performs a highly accurate writing operation while suppressing the variation of the influence caused by the ozone introduction.
Another object of the present invention is to provide a charged particle beam writing method that stably performs a highly accurate writing operation while suppressing the variation of the influence caused by the ozone introduction.
Other challenges and advantages of the present invention are apparent from the following description.